(Chapter 19) Footnote 3:

The verb regularly used by Vesalius for the change from bone into cartilage, degenerant, is not translated literally here. “Change” is too neutral, as throughout the Fabrica Vesalius implicitly favors hard tissue over soft. The Latin verb contains the sense of reversion to a former less highly developed condition ( OLD, 2), making “revert” a possible alternative. Cf. Bk. 2 ch. 37: “Cartilages are not attached to the ribs themselves, but as has been said the very bones of the ribs revert into them.”